The use of parts comprised of a polymeric substrate such as plastic having an electrodeposited coating on all or portions of the surfaces thereof has received widespread commercial acceptance for a variety of utilitarian and decorative purposes including automobile trim components. Various processes and techniques have heretofore been used or proposed for applying such metallic coatings on polymeric substrates including pretreatments to activate the plastic surface followed by the deposition of an electroless metal deposit whereafter the part can be subjected to further electroplating operations. More recently, so-called "plateable" plastics have been developed which incorporate conductive filler materials such as graphite to enable direct electroplating of the substrate without necessitating the prior surface activation and electroless plating steps. In addition to the problems associated with such prior art processes including high costs, complexity of the process, low efficiency in the electroplating cycle and necessity of waste treatment of the several intervening rinse treatments, a further problem has arisen as a result of the loss of adhesion between the overlying metallic layer and the substrate as evidenced by blistering when such plated parts are subjected to elevated temperatures such as may occur during the baking or curing cycle of painted plastic parts as well as during service.
In accordance with the guidelines established by the American Society of Electroplated Plastics, a minimum thickness of copper deposit on the plastic substrate has been specified depending on the severity of service in order for such plated plastic articles to pass thermocycle testing. According to ASEP guidelines, a copper deposit of at least about 5 to about 10 micrometers is specified for minimum service with copper thicknesses of as high as about 15 to about 20 micrometers for heavy duty service. The deposition of the copper plate is primarily accomplished by a conventional acid copper plating solution usually containing primary and secondary brightening agents to provide a conductive decorative copper deposit. Subjection of plastic substrates having an electroless plate on the surfaces thereof directly to such acid copper plating operations has frequently resulted in a burn-off of the electroless plate at the contact points and a loss of adhesion of the copper plate to the substrate. In order to avoid burn-off, it has heretofore been necessary to substantially reduce the initial voltage and amperage of the acid copper electroplating step to provide for a progressive build-up of the copper deposit which has resulted in a substantial loss in efficiency accompanied by excessive plating times.
In an effort to overcome the foregoing problem it has also been proposed to apply a nickel strike employing a Watts nickel bath over the electroless plated plastic part to build-up a conductive nickel plate of a thickness usually up to about 0.0001 inch. This proposal also has the disadvantages that it is necessary to subject the part with the nickel strike thereon to usually two intervening water rinse treatments before entering the subsequent acid copper plating solution necessitating waste disposal treatment of the rinse solutions in order to enable them to be harmlessly discharged to waste. Additionally, the cost of such nickel baths are comparatively high and the nickel plate or strike deposited cannot be included to fulfill the requirements of minimum copper deposits in accordance with the guidelines established by the ASEP.
Alternatively, it has heretofore been proposed to employ a copper pyrophosphate electrolyte for applying a copper strike to the substantially non-conductive plastic substrate such as an electroless plating deposit to render the part suitable for further copper plating employing a conventional acid copper electrolyte. Unfortunately, such copper pyrophosphate electrolytes are difficult to control to consistently obtain a uniform copper strike and the cost of the bath is relatively high. Additionally, at least two intervening rinses are required between the copper strike and subsequent acid copper decorative plate necessitating further costly waste treatment of the rinse solution.
The problems and cost disadvantages associated with prior art techniques for electroplating plastics are overcome in accordance with the process of the present invention whereby an adherent copper strike is applied to a substantially non-conductive substrate which contributes toward the minimum copper guidelines as established by the ASEP and whereby the part can be directly transferred from the strike bath to the decorative acid copper electrolyte without necessitating any intervening rinse treatments. The process of the present invention is further characterized by the economy of the conditioning and copper strike bath, the ease of control of the process to achieve consistently uniform and adherent copper deposits and wherein any dragout from the strike solution to the subsequent decorative acid copper solution effects a replenishment of the latter providing for further economies and a conservation of chemical constituents.